The critical role of Complement in host defense against experimental bacterial infections by Streptococcus pneumoniae has been documented. The alternative pathway of complement activation provides the primary host defense in nonimmune animals, whereas, the classical pathway is necessary for optimal effect of pneumococcal antibodies. Additional studies have concerned the role of complement in mediating certain aspect of sepsis, such as the respiratory distress syndrome, and the role of the spleen in clearance of pneumococcal bacteremia.